In one exhaust timing control device according to the prior art, a rotary type valve member is disposed in the edge portion of an exhaust passage at the side of a cylinder head which opens into the inner circumferences of a cylinder. The aforementioned valve member is formed into a drum shape with its circumference extending along the inner circumference of the cylinder and is axially divided into two halves. This valve member is rotatably disposed in the recess, which recess is formed in the edge portion of the exhaust passage at the side of the cylinder head, so that its axial direction of rotation intersects the axial direction of the cylinder at a right angle. By turning the aforementioned valve member, therefore, the edge portion of the circumference thereof which faces the inner circumference of the cylinder is shifted in the axial direction of the cylinder so that the height of the opening of the exhaust passage is varied to change the exhaust timing.
However, since the aforementioned conventional exhaust timing control device for a two-cycle engine has its valve member formed into a drum shape, the valve member has its diameter made remarkably larger at its ends than at its intermediate portion so that the valve member itself tends to be larger than optimal. As a result, the recess to receive the aforementioned valve member is accordingly enlarged so that the thickness of the cylinder body is accordingly reduced, and this involves a reduction in strength. Especially the portion of the cylinder body which is sandwiched between the valve member and the inner circumference of the cylinder is thinned, because the valve member is of the rotary type, thereby further to invite a disadvantage in strength. There results the risk that the cylinder may be thermally deformed. In the aforementioned conventional exhaust timing control device for the two-cycle engine, moreover, the valve member is so arranged that its axis of rotation intersects the axial direction of the cylinder. The valve member, which has its drum shape divided at its center portion into the right and left halves, is inserted, when it is to be assembled in the receiving portion of the cylinder body from the side of the cylinder in the direction to intersect the axial direction of the cylinder. In the case of a multi-cylinder engine, therefore, the respective cylinders have to be made so separate that the adjoining cylinders may not constitute obstacles upon insertion and assembly of the valve member, frustrating the design of an integrated multi-cylinder body.
It is therefore an object of the present invention to provide such an exhaust timing control device for a two-cycle engine which has no thinned portion in a cylinder body, which is free from deformation from thermal stress, which can be assembled without difficulty, and which can adopt a multi-cylinder body.